In conventional resistance welding, for example spot welding, generally, the welding strength between the welded members is largely dependent upon the electrode pressure, the size and shape of the electrode tips, the pressure-related wear on the electrodes, and the quality or wear of the welded members. Therefore, it is impossible to obtain consistent welding quality even if the electrode pressure, welding current, and welding time are maintained at constant levels. The term "electrode pressure" used herein refers to the pressure between the electrodes while pinching the welded members.
To cope with this problem, various monitoring systems have previously been devised, such as electrode voltage systems, electrode tip resistance systems, and ultrasonic wave systems. However, since these monitoring systems can only roughly determine the quality of the welded portions after completion of the welding and their respective fields of application differ, they cannot positively ensure the quality of the welded portions. (The term "quality of the weld" refers to the size and penetration rate of the nugget formed at the weld site, the tensile and shearing strengths of the welds thereby provided, and the like.) Accordingly, even when the various prior art resistance welding systems or various monitoring systems are used in combination, there can arise cases in which the quality of weld is poor, making it necessary to repair the weld or discard the product, depending upon the case.
Thus, a controlling system has recently been proposed to overcome this difficulty which automatically ensures the quality of the resistance weld in the process of welding by controlling the electrode pressure or both the electrode pressure and the welding current so that the voltage between the electrodes (including the electrode tips in the case of spot welding) pinching the welded members can be adjusted in accordance with a predetermined reference voltage curve, the instantaneous voltage being chosen to provide a high-quality weld, on the basis that the above voltage has a close relation to the quality of the weld. However, with this system alone, it is impossible to control the size of the nugget of the weld optimally at all times. For instance, in th case where the electrodes are worn by pressure, the diameter of the nugget will become excessively large resulting in an excessively strong weld and greater power consumption than necessary. On the other hand, depending upon the welding conditions, such as the condition of contact surfaces of the welded members, it may happen that, even if the electrode voltage varies in accordance with the reference voltage curve, the required diameter of nugget cannot be obtained due to insufficient current path area.
The present invention is intended to solve the above problem and to automatically ensure the desired quality of the weld without excess and deficiency at all times during the welding process.